This paper deals with a comparative study on the mechanical, acoustic, and electromagnetic properties of a novel class of auxetic (negative Poisson's ratio) rigid polyurethane (PU) foam with a magnetorheological (MR) fluid coating. An auxetic solid expands in all directions when pulled in only one, thus behaving in an opposite manner compared to 'classical' solids. When compared with the conventional PU foam, the auxetic PU foam shows enhanced crashworthiness properties and increased sound absorption characteristics (at low frequencies). Samples of auxetic foam coated with MR fluid are examined in this work. While the tensile mechanical properties of the MR fluid-coated samples are affected mainly by surface effects, the acoustic absorption characteristics show almost constant values beyond the cut-off frequency level of the original uncoated auxetic foam. With regard to the foams' electromagnetic properties, the auxetic structure and MR coating cause an increase in the refractive index and loss factor compared to the conventional foams. These electromagnetic effects are due to the presence of a high relative density and metal particulates in the foam material.

The cellular composite's unit cell is based on the common honeycomb configuration, but includes auxetic and nonauxetic variants. Formulas that describe the essential electromagnetic and structural properties of the honeycomb are described, and a multidisciplinary design procedure outlined. The effects of the manufacturing process on design methodology are also discussed.

The paper describes new concepts of flexible polyurethane auxetic (negative Poisson’s ratio) foams with embedded carbonyl dispersions. Compared to conventional flexible PU foams, this type of cellular solid exhibit higher compliance under impact loading and increased dielectric loss factors in the X-band between 12 and 18 GHz. Results from mechanical tensile tests and rectangular wave guide experiments are shown and discussed in view of possible applications on sandwich structures typical of Salisbury screens or microwave absorbers.

Abstract In this work a combined analysis of the out-of-plane mechanical and the electromagnetic properties of auxetic re-entrant honeycombs is performed. Experimental and numerical simulations are carried out to evaluate the correlation between the anisotropicity of the transverse mechanical properties (shear and compressive modulus) and the permittivity tensor of auxetic honeycombs. The results are evaluated to assess the feasibility of this kind of cellular solid for electromagnetic absorption and window applications with high structural integrity performance.

A bidirectional electromagnetic screen is proposed and demonstrated with properties that can be switched between high reflection, absorption and transmission over similar to10 ns. The screen comprises frequency selective surfaces loaded with pin diodes, which under partial biasing conditions provide the loss mechanism for the absorption state of the screen.

A mechanically adaptive Salisbury screen is used to show that an adaptive screen can circumvent the bandwidth/thickness limitations of passive thin materials. The electronic analogue of the mechanical screen is described and the impedance characteristics of the electronic adaptive elements determined for single and multi-adaptive layered configurations.

The electromagnetic consequences of placing a dielectric coating at the edge of a conducting plane are discussed. An empirically based coating design strategy is formulated that uses asymptotic RCS data and surface wave attenuation data to determine those properties of the coating that will effect a reduction in monostatic RCS at obtuse incidence angles and H-polarisation (magnetic field polarised parallel to the edge).

In this work a combined analysis of the out-of-plane mechanical and dielectric properties of auxetic re-entrant honeycombs is performed. Experimental and numerical simulations are carried out to evaluate the correlation between the anisotropicity of the transverse mechanical properties (shear and compressive modulus) and the permittivity tensor of general and auxetic (Negative Poisson's ratio) honeycombs. Different numerical and experimental techniques have been applied to obtain the mechanical and dielectric properties of genral and auxetic honeycombs versus the core material and unit cell geometry parameters. The results are evaluated to assess the feasibility of this kind of cellular solid for electromagnetic screen applications with high structural integrity performance.

A thin tapered absorber is described that is capable of effecting wide band absorption of electromagnetic waves at oblique incidence. The absorption mechanism for the taper is shown to be inherently wide band and seems not to be constrained by the bandwidth/reflectivity limitations that apply to thin planar materials. Predicted reflectivity data for the tapered absorbing layer are shown.

A technique is described for measuring the RCS of 2D (two-dimensional) diffracting sources. The measured and predicted diffraction RCS of a conducting halfplane edge and a gap in an otherwise infinite conducting plane are presented. The technique may be used to measure the diffraction coefficient of an impedance discontinuity.

Dielectric honeycombs are cellular materials often used in applications that require structural and electromagnetic characteristics, e.g., in LO (low observable) and radome components, A re-entrant (or auxetic) honeycomb is a cellular material with structural properties that are superior to those of a conventional honeycomb. By employing the finite-difference time-domain (FDTD) technique with periodic boundary conditions, the electromagnetic properties of re-entrant honeycombs are determined and compared to those of a conventional honeycomb. Re-entrant honeycombs are shown to have substantially superior electromagnetic properties. Measured permittivity data are used to substantiate the conclusions based on predicted FDTD data. The use of re-entrant honeycombs, rather than conventional honeycombs, in LO and radome applications can yield improved structural and electromagnetic performance.

The effective permittivity of a dielectric honeycomb can be represented by a diagonal permittivity tensor. In the paper, an empirical equation is derived using the FD-TD (finite difference-time domain) method, which defines the diagonal components of the complex permittivity in terms of the honeycomb material permittivity and the fraction of honeycomb material per unit cell. The impact is investigated of electrically large honeycomb cells on effective permittivity failure. The manner in which the effective permittivity fails proves to favour a honeycomb orientation such that the direction of an incident plane wave's Poynting vector is parallel with the honeycomb's axis. The consequences are considered of an invalid effective permittivity on radar cross-section and planar reflectivity.